Process for preparing bisallylboranes and nonaromatic boronic acids

ABSTRACT

A process for preparing bisallylboranes of the formula (I) by reacting a diene with sodium borohydride in the presence of an oxidant: 
                 
 
in an inert solvent, with the borane generated in situ reacting selectively with the diene to form the bis(allyl)borane of the formula (I) and the substituents R 1  to R 6  having the following meanings:
         R 1 14 R 6  are H, aryl or substituted or unsubstituted C 1 -C 4 -alkyl or two radicals R may be closed to form a cyclic system.       

     As oxidant, it is possible to use, for example, alkyl halides or dialkyl sulfates. In a particularly preferred embodiment, the diene used is 2,5-dimethylhexa-2,4-diene (R 1 , R 2 , R 5 , R 6 =methyl, R 3 , R 4 =H).

This application is a Divisional application of application Ser. No.10/236,749, filed Sep. 6, 2002, now U.S. Pat. No. 6,706,925, thecontents of which are hereby incorporated by reference.

Aromatic boronic acids have become indispensible in the synthesis ofcomplex organic molecules. They make it possible to produce manystructures which are otherwise obtainable only with difficulty.

Many examples of such Suzuki reactions are reported in the chemicalliterature. Among these, C—C coupling reactions in which aliphatic orolefinic boronic acids are used are relatively rare.

One reason for this is the usually challenging synthesis of newnonaromatic boronic acids which are of interest for research. Suchboronic acids would allow the introduction of interesting substructureswhich cannot be achieved or achieved only with difficulty by means ofclassical synthesis.

Aliphatic and olefinic boronic acids are usually prepared by means ofhydroboration of alkenes and alkynes or else via conversion ofhaloolefins into Grignard compounds and addition of the resultingorganomagnesium compounds onto esters of boric acid. However, thesereactions frequently display only low chemoselectivities andregioselectivities. Examples are the reaction of catecholborane withstyrenes or the reaction of alkylmagnesium halides with boric esters.

The handling of boranes on an industrial scale, in particular, is oftenan obstacle because of their hazardous properties.

The present invention describes a simple, direct route to alkylboronicand alkenylboronic acids via the corresponding bis(allyl)boranes whichgives good yields and can be carried out in a single vessel.

The present invention accordingly provides a process for preparingbis(allyl)boranes of the formula (I) by reacting a diene with sodiumborohydride in the presence of an oxidant in an inert solvent, with theborane generated in situ reacting selectively with the diene to form thebis(allyl)borane of the formula (I). The first step of the reactiongenerates borane which dimerizes in the absence of the diene to formdiborane but reacts with the diene to form the correspondingbis(allyl)borane highly selectively.

This route to bis(allyl)boranes is novel and was thus not to be expectedsince the reaction of borane with dienes is described in the literatureas largely unselective, giving predominantly borolanes as main products(Dokl. Akad. Nauk. SSSR, Ser. Khim. 1964, 155, 141; Izv. Akad. Nauk.SSSR, Ser. Khim. 1965, 2111). Bis(allyl)boranes have hitherto beenprepared in the literature via thermal disproportionation reactions.

In a preferred embodiment; the diene used is 2,5-dimethylhexa-2,4-diene(R¹, R², R⁵, R⁶=methyl, R³, R⁴=H).

The alkene or alkyne is added to the bis(allyl)borane which has beengenerated in this way and is reacted therewith. This reaction proceedshighly regioselectively and chemoselectively. The alkylbis(allyl)borane(V) or alkenylbis(allyl)borane (III) is selectively oxidized in the samevessel to the corresponding bisallyl boronate. The bis(allyl) ester canbe isolated as such or converted into a derivative.

The bis(allyl)boranes and bis(allyl) esters produced can be used in thesame vessel in C—C coupling reactions, for example Suzuki reactions.Isolation of the boronic acid can be circumvented. This is particularlyuseful when the boronic acid is very sensitive to oxidation, heat orhydrolysis.

In the first reaction step, diborane is generated in situ from sodiumborohydride by reaction with an oxidant (Ox), e.g. with substituted orunsubstituted C₁-C₈—, in particular C₁-C₄-alkyl halides or dialkylsulfates, preferably with alkyl iodides or bromides or dialkyl sulfates,particularly preferably with dimethyl sulfate, diethyl sulfate, benzylbromide or iodoethane, in an inert solvent at temperatures of from −20to +30 ° C., preferably from −5 to +10° C., and this then reactsselectively with the diene present in the reaction mixture to form thecorresponding bis(allyl)borane I.

Suitable solvents are, for example, various ethers or hydrocarbons, inparticular C₁-C₁₀ hydrocarbons or mixtures thereof, preferably etherssuch as end-proteceted oligoglycol or polyglycol ethers or THF,particularly preferably diglyme, which are inert toward the reactants.

The radicals R¹-R⁶ are H, aryl or C₁-C₄-alkyl, substituted orunsubstituted, and may be closed to form a cyclic system, e.g. via theradicals R¹ and R⁵ to form a six-membered ring, preferably H and methyl.In a particularly preferred embodiment, 2,5-dimethylhexa-2,4-diene (R¹,R², R⁵, R⁶=methyl, R³, R⁴═H) are employed. The diene is used in anamount of from 1 to 10 molar equivalents based on sodium borohydride,preferably 2-3 molar equivalents. The concentration of thebis(allyl)borane formed is from 0.1 to 5 mol/l, preferably from 0.5 to 2mol/l.

The present invention further provides a process for preparing boronicacids by reaction of a diene with sodium borohydride in the presence ofan oxidant to form the corresponding bis(allyl)borane of the formula (I)and further reaction of the borane (I) with an appropriate alkene (II)or alkyne (IV) to give the alkylbis(allyl)borane (III) oralkenylbis(allyl)borane (V) which is oxidized directly in the presenceof an oxidant to form the corresponding bisallyl alkylboronate oralkenylboronate and, if desired, subsequent conversion into aderivative.

The bis(allyl)borane (I) produced in this way is reacted in the samevessel with a substituted alkene (II) or alkyne (IV) at temperatures inthe range from −10 to 100° C., preferably from 0 to 50° C., particularlypreferably from 0 to 25° C., to form the alkylbis(allyl)borane (III) oralkenylbis(allyl)borane (V), which can advantageously be employed in C—Ccoupling reactions, in particular in Suzuki coupling reactions.

The radicals R⁷ to R¹² are, in particular, aryl, substituted orunsubstituted, ailkyl-(C₁-C₈), which may be branched and/or substituted,alkoxy-(C₁-C₈), acyloxy-(C₁-C₈), O-phenyl, fluorine, chlorine. NO₂, NH₂,NHalkyl-(C₁-C₈), Nalkyl₂-(C₁-C₈), CN, CHO, SO₃H, SO₃R, SO₂NH₂,SO₂N(alkyl-(C₁-C₈))₂, SO₂-alkyl-(C1-C₈), COO-alkyl-(C₁-C₈), CONH₂,CO-alkyl-(C₁-C₈), NHCHO, CF₃, 5-membered heteroaryl or 6-memberedheteroaryl. Two radicals can also form a cyclic system which may containheteroatoms.

The range of alkenes and alkynes which can be used is very wide. Theexamples given for substituents are only an illustrative selection fromthe possible range and do not restrict the process of the invention tothese.

To avoid selectivity problems in the oxidation, for example in theoxidation of bis(cyclohexyl)boranes to bis(cyclohexyloxy)boronic acidsby means of N-oxides as described in the literature (Synthesis, 1988,103), keto compounds are used according to the invention for theoxidation in the third reaction step after the hydrolysis. It ispossible to use, for example, formaldehyde, acetone, glyoxal, diacetyl,preferably formaldehyde and diacetyl.

The oxidation takes place at temperatures of from 0 to 100° C.,preferably from 10 to 40° C. The keto compounds are added in a molarratio of 1-2 based on sodium borohydride used.

The oxidation of alkenylbis(allyl)boranes (V) by means of formaldehydegives boronic esters of the formula (VI), while that by means ofdiacetyl gives esters of the formula (VII).

The boronic esters which result from the oxidation of thealkylbis(allyl)boranes (III) have an analogous structure, with the onlydifference being that the alkyl substituent on the boron is different.

The boronic esters produced in this way can be converted into theappropriate derivatives by known methods. For example, hydrolysis givesthe free boronic acids (Q1, Q2=OH), reaction with pinacol gives thecorresponding pinacol esters (Q1, Q2=—OC(CH₃)₂—C(CH₃)₂O—) and reactionwith diethanolamine gives the corresponding diethanolamine esters (Q1,Q2=—OCH₂CH₂NHCH₂CH₂O—).

In principle, any desired derivatives can be prepared without problems,for example simple alkyl esters (Q1, Q2=Oalkyl) or cyclic esters derivedfrom ethylene glycol or catecholborane.

The process described offers, in particular, the opportunity of reactingthe bis(allyl)boranes (III) and (V) or the ester derivatives (VI) and(VII) (including the corresponding alkylboronic esters from thebis(allyl)boranes (III)) produced in situ with any suitable substratesin a Suzuki coupling. The substrates have to have a leaving group LG,with LG being, in particular, I, Br, Cl or OSO₂CF₃. Examples ofsubstrates are bromoaromatics or iodoaromatics and also bromoolefins oriodoolefins.

EXAMPLE 1 Preparation of di(1-isopropyl-3-methylbut-2-enyl)borane

NaBH₄ (0.757 g, 20 mmol), 2,5-dimethylhexa-2,4-diene (7.10 ml, 50 mmol)and anhydrous diglyme (20 ml) were placed in a baked 100 ml flask underan Ar atmosphere and cooled in an ice bath. (MeO)₂SO₂ (1.90 ml, 20 mmol)was slowly added while stirring vigorously to the reagent slurryobtained after cooling, with the reaction temperature being kept below5° C. The addition was accompanied by vigorous gas evolution andclarification of the reaction mixture. The solution formed was stirredat 0° C. for 3 hours, giving a thick suspension ofdi(1-isopropyl-3-methylbut-2-enyl)borane which was used directly in thehydroboration step.

EXAMPLE 2 General method A2-[(E)-2-Phenyl-1-ethenyl]-1,3,6,2-dioxazaborocane

Phenylacetylene (2.20 ml, 20 mmol) was added slowly (˜20 min) to asuspension of freshly prepared di(1-isopropyl-3-methylbut-2-enyl)borane,with the reaction temperature being below 5° C. The mixture formed wasstirred at 0° C. for 1 hour, slowly quenched with H₂O (3 ml) (with somegas evolution occurring) and stirred at RT for 0.5 hour, after which asolution of formaldehyde (1.50 ml, 20 mmol, 37% strength by weightsolution in water) was added all at once (exothermic reaction, coolingbath necessary). The reaction mixture was stirred at RT for 24 hours andthen diluted with EtOAc (40 ml). After separation of the layers, theorganic phase was dried briefly (Na₂SO₄), transferred into a flask withdiethanolamine (2.31 g, 22 mmol) and evaporated under reduced pressure(25 mm of Hg, then 0.5 mm of Hg, heating bath 50° C., then 80° C.). Thesolid residue obtained after evaporation was recrystallized from MeCN,giving the product (3.34 g, 15.38 mmol, 77% yield) in the form ofcolorless needles, m.p. 194-195° C.; IR (KBr) 3000 br, 1622, 1598, 1573,1494, 1469, 1454, 1277, 1242, 1206 cm⁻¹; ¹H-NMR (200 MHz, DMSO-d₆)7.38-7.23 (complex, 4H), 7.17-7.14 (m, 1H), 6.84 (br. s, 1H), 6.65 (d,1H, J=18.1 Hz), 6.23 (d, 1H, J=18.1 Hz), 3.80-3.61 (complex, 4H),3.08-2.90 (m, 2H), 2.81-2.71 (m, 2H); ¹³C-NMR (75.4 MHz, methanol-d₄)140.9, 139.7, 129.6, 129.5, 128.0, 127.3, 64.1, 51.7; MS EI (m/e,relative intensity) (M+ 217).

EXAMPLE 3 2-[(Z)-1,2-Diphenyl-1-ethenyl]-1,3,6,2-dioxazaborocane

This compound was prepared by the general method A fromdiphenylacetylene (3.56 g, 20 mmol), which was added as a solid to thereaction, and was isolated in a yield of 66% (3.89 g, 13.27 mmol) in theform of colorless crystals, m.p. 227-228.5° C.; IR (KBr) 3222, 3076,2859, 1595, 1493, 1270, 1198, 1132, 1102, 1070, 1050 cm⁻¹; ¹H-NMR (200MHz, DMSO-d₆) 7.30-6.90 (m, 8H), 6.90-6.80 (m, 2H), 6.71, (s, 1H), 6.58(br. s, 1H), 3.79-3.65 (m, 2H), 3.65-3.45 (m, 2H), 3.00-2.65 (m, 4H);¹³C-NMR (75.4 MHz, methanol-d₄) 146.3, 140.2, 133.9, 130.6, 129.5,129.4, 128.7, 127.1, 126.3, 63.9, 52.0;

MS EI (m/e, relative intensity) (M+ 293)

EXAMPLE 4 2-[2-(1,1,1-Trimethylsilyl)ethyl]-1,3,6,2-dioxazaborocane

This compound was prepared by the general method A fromvinyltrimethylsilane (2.93 ml, 20 mmol) with the reaction mixture beingstirred for a further 2 hours at RT before quenching with water and wasisolated in a yield of 55% (2.35 g, 10.92 mmol) in the form of colorlessneedles; m.p. 185-186° C.; IR (KBr) 3090, 2955, 2897, 1461, 1421, 1353,1296, 1244, 1216, 1159, 1111, 1065, 963 cm⁻¹; ¹H-NMR (200 MHz, DMSO-d₆)6.52 (br. s, 1H), 3.73-3.60 (m, 2H), 3.60-3.50 (m, 2H), 3.05-2.88 (m,2H), 2.69-2.62 (m, 2H), 0.34-0.20 (m, 2H), 0.20-0.04 (m, 2H), -0.09 (s,9H); ¹³C-NMR (75.4 MHz, methanol-d₄) 63.8, 52.2, 11.4, -1.64;

MS EI (m/e, relative intensity) (M+ 215)

EXAMPLE 5 2-(2,3-Dihydro-1H-2-indenyl)-1,3,6,2-dioxazaborocane

This compound was prepared by the general method A from indene (2.33 ml,20 mmol) with the reaction mixture being stirred for a further 2 hoursat RT before quenching with water and was isolated in a yield of 48%(2.24 g, 9.69 mmol) in the form of colorless needles; m.p. 216-219° C.(decomp.); IR (KBr) 3091 br, 2931, 2887, 1482, 1456, 1274, 1244, 1216,1112, 1067 cm⁻¹; ¹H-NMR (300 MHz, DMSO-d₆) 8.28-8.17 (m, 2H), 8.17-8.06(m, 2H), 7.93 (br. s, 1H), 4.91-4.80 (m, 2H), 4.80-4.70 (m, 2H), 4.45(d, 1H, J=6.9 Hz), 4.21-4.05 (m, 2H), 3.98-3.82 (m, 4H), 3.76-3.67 (m,2H); ¹³C-NMR (75.4 MHz, methanol-d₄) 147.2, 126.4, 124.9, 64.0, 52.3,36.8;

MS EI (m/e, relative intensity) (M+ 231)

EXAMPLE 6 2-[2-(9H-9-Carbazolyl)ethyl]-1,3,6,2-dioxazaborocane

This compound was prepared by the general method A from N-vinylcarbazole(3.87 g, 20 mmol), which was added as a solid, with the reaction mixturebeing stirred for a further 8 hours at RT before quenching with waterand was isolated using MeOH for the recrystallization in a yield of 67%(4.14 g, 13.43 mmol) in the form of colorless platelets; m.p. 241-244°C. (decomp.); IR (KBr) 3102 br, 1593, 1485, 1452, 1335, 1328, 1272,1236, 1182, 1104, 1061 cm⁻¹; ¹H-NMR (200 MHz, DMSO-d₆) 8.11 (d, 2H,J=7.7 Hz), 7.53 (d, 2H, J=8.1 Hz), 7.42 (td, 2H, J=6.7, 1.0 Hz), 7.14(td, 2H, J=7.7, 0.7 Hz), 7.00 (br. s, 1H), 4.32-4.19 (m, 2H), 3.85-3.60(m, 4H), 3.20-3.00 (m, 2H), 2.90-2.70 (m, 2H), 0.85-0.70 (m, 2H);¹³C-NMR (75.4 MHz, methanol-d₄) 139.6, 125.3, 121.9, 120.2, 118.0,109.1, 62.4, 50.7, 40.9;

MS EI (m/e, relative intensity) (M+ 308)

EXAMPLE 7 General Method B:4,4,5,5-Tetramethyl-2-[(E)-1-octenyl]-1,3,2-dioxaborolane

1-Octyne (2.95 ml, 20 mmol) was added slowly (˜20 min) to a suspensionof freshly prepared di(1-isopropyl-3-methylbut-2-enyl)borane, with thereaction temperature being below 5° C. The mixture formed was stirred at0° C. for 1 hour, slowly quenched with H₂O (3 ml) (with some gasevolution occurring) and stirred at RT for 0.5 hour, after which asolution of formaldehyde (1.50 ml, 20 mmol, 37% strength by weightsolution in water) was added all at once (exothermic reaction, coolingbath necessary). The reaction mixture was stirred at RT for 1 hour andthen admixed with pinacol (2.60 g, 22 mmol). After stirring at RT for 24hours, the mixture was diluted with H₂O (40 ml) and extracted withheptane (40 ml), after which the organic extract was washed with H₂O(5×25 ml), dried (Na₂SO₄) and evaporated under reduced pressure. Theresidue obtained was carefully distilled under reduced pressure (bulbtube), giving 3.19 g of the product (13.39 mmol, 67% yield) in the formof a colorless liquid, b.p. 70-80° C. (0.15 mm of Hg); IR (film) 2928,1639, 1363, 1319, 1146 cm⁻¹; ¹H-NMR (300 MHz, acetone-d₆) 6.55 (dt, 1H,J=18.0, 6.6 Hz), 5.35 (dt, 1H, J=18.0, 1.5 Hz), 2.17-2.09 (m, 2H),1.50-1.20 (m, 8H), 1.21 (s, 12H), 0.90-0.85 (m, 3H); ¹³C-NMR (75.4 MHz,acetone-d₆) 155.0, 83.6, 36.5, 32.5, 29.7, 29.2, 25.2, 23.3, 14.4;

MS EI (m/e, relative intensity) (M+ 238).

EXAMPLE 8 4,4,5,5-Tetramethyl-2-[(Z)-1-methyl-2-phenyl-1-ethenyl]-1,3,2-dioxaborolane (Main Component) and4,4,5,5-tetramethyl-2-[(Z)-1-phenyl-1-propenyl]-1,3,2-dioxaborolane(Secondary Component)

These compounds were prepared by the general method B from1-phenylprop-1-yne (2.50 ml, 20 mmol) and isolated in a yield of 70%(3.41 g, 13.97 mmol) in the form of a colorless liquid which, accordingto ¹H-NMR, was an 83:17 isomer mixture; b.p. 75-90° C. (0.2 mm of Hg);¹H-NMR (300 MHz, acetone-d₆) 7.42-7.10 (complex, 5H), 7.20 (br. s,0.83H) and 6.70 (q, 0.17H, J=7.2 Hz), 1.94 (d, 2.49H, J=1.8 Hz) and 1.72(d, 0.51H, J=7.2 Hz), 1.28 (s, 9.96H) and 1.23 (s, 2.04H); ¹³C-NMR (75.4MHz, acetone-d₆, main component (secondary component)) 143.1(143.1),138.8(141.0), 130.2(130.0), 129.1(128.6), 128.2(126.7), 84.1(84.3),25.3(25.2), 16.2(16.2).

EXAMPLE 9 4,4,5,5-Tetramethyl-2-phenethyl-1,3,2-dioxaborolane

This compound was prepared by the general method B from styrene (2.29ml, 20 mmol) with the reaction mixture being stirred for a further 2hours at RT and was isolated in a yield of 71% (3.28 g, 14.13 mmol) inthe form of a colorless liquid, b.p. 70-80° C. (0.1 mm of Hg); IR (film)2979, 1372, 1322, 1145 cm⁻¹; ¹H-NMR (300 MHz, acetone-d₆) 7.27-7.15 (m,4H), 7.15-7.09 (m, 1H), 2.68 (t, 2H, J=8.0 Hz), 1.18 (s, 12H), 1.03 (t,2H, J=8.0 Hz); ¹³C-NMR (75.4 MHz, acetone-d₆) 145.4, 129.0, 128.8,126.3, 83.7, 30.8, 25.2;

MS EI (m/e, relative intensity) (M+ 232)

EXAMPLE 102-(3,3-Diethoxypropyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

This compound was prepared by the general method B from acrolein diethylacetal (3.05 ml, 20 mmol) with the reaction mixture being stirred for afurther hours at RT and was isolated in a yield of 63% (3.26 g, 12.63mmol) in the form of a colorless liquid; b.p. 65-80° C. (0.25 mm of Hg);IR (film) 2977, 2932, 2880, 1371, 1325, 1147, 1127, 1061 cm⁻¹; ¹H-NMR(200 MHz, acetone-d₆) 4.39 (t, 1H, J=5.7 Hz), 3.65-3.35 (m, 4H),1.65-1.55 (m, 2H), 1.20 (s, 12H), 1.12 (t, 6H, J=7.0 Hz), 0.74 (t, 2H,J=8.0 Hz); ¹³C-NMR (125.8 MHz, acetone-d₆) 104.8, 83.5, 61.4, 28.8,25.1, 15.7;

MS EI (m/e, relative intensity) (M+ 258)

EXAMPLE 114-[2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)ethyl]-1,3-dioxolan-2-one

This compound was prepared by the general method B from4-vinyl-1,3-dioxolan-2-one (1.92 ml, 20 mmol) with the reaction mixturebeing stirred for a further 2 hours at RT and using EtOAc (80 ml) in theextraction step in a yield of 77% (3.72 g, 15.37 mmol) in the form of acolorless oil; b.p. 105-120° C. (0.25 mm of Hg); IR (film) 2980, 2933,1805, 1381, 1328, 1166, 1145, 1066 cm⁻¹; ¹H-NMR (300 MHz, acetone-d₆)4.81-4.71 (m, 1H), 4.59 (td, 1H, J=8.1, 0.6 Hz), 4.15 (dd, 1H, J=8.1,6.9), 1.86-1.76 (m, 2H), 1.22 (s, 12H), 0.85-0.76 (m, 2H); ¹³C-NMR (75.4MHz, acetone-d₆) 155.7, 84.0, 79.1, 69.9, 28.9, 25.23, 25.16;

MS EI (m/e, relative intensity) (M+ 242)

EXAMPLE 12 General Method C:trans4,5-Dimethyl-2-[(E)-2-phenyl-1-ethenyl]-4,5-di[(E)-1,1,4-trimethyl-2-pentenyl]-1,3,2-dioxaborolane

Phenylacetylene (2.20 ml, 20 mmol) was added slowly (˜20 min) to asuspension of freshly prepared di(1-isopropyl-3-methylbut-2-enyl)borane,with the reaction temperature being kept below 5° C. The mixture formedwas stirred at 0° C. for 1 hour, after which 2,3-butanedione (1.93 ml,22 mmol) was added all at once (exothermic reaction, cooling bathrequired). After stirring at RT for 12 hours, the reaction mixture wasevaporated under reduced pressure, diluted with H₂O (40 ml) andextracted with heptane (40 ml). The organic extract was washed with H₂O(3×20 ml), dried (Na₂SO₄) and evaporated, after which the residueobtained was distilled under reduced pressure (bulb tube), giving 6.06 g(14.34 mmol, 72% yield) of the product in the form of a colorlessviscous oil; b.p. 140-155° C. (0.1 mm of Hg); IR (film) 2960, 1626,1578, 1451, 1388, 1352, 1326, 1209, 1082 cm⁻¹; ¹H-NMR (200 MHz,acetone-d₆) 7.63-7.58 (m, 2H), 7.45 (d, 1H, J=18.4 Hz), 7.46-7.35 (m,3H), 6.27 (d, 1H, J=18.4 Hz), 5.71 (dd, 2H, J=15.8, 1.1 Hz), 5.35 (dd,2H, J=15.8, 6.8 Hz), 2.30-2.15 (m, 2H), 1.53 (s, 6H), 1.12 (s, 6H), 1.11(s, 6H), 0.94 (d, 6H, J=6.7 Hz), 0.93 (d, 6H, J=6.7 Hz); ¹³C-NMR (75.4MHz, acetone-d₆) 150.4, 138.6, 135.8, 134.8, 129.9, 129.6, 128.0, 92.9,46.6, 32.4, 26.2, 26.1, 22.95, 22.88, 19.4;

MS EI (m/e, relative intensity) (M+ 422)

EXAMPLE 13trans-2-[(E)-2-(1,1,1-Trimethylsilyl)-1-ethenyl]-4,5-dimethyl-4,5-di[(E)-1,1,4-trimethyl-2-pentenyl]-1,3,2-dioxaborolane

This compound was prepared by the general method C fromtrimethylsilylacetylene (2.83 ml, 20 mmol) and was isolated in a yieldof 59% (4.91 g, 11.73 mmol) in the form of a colorless viscous oil, b.p.120-130° C. (0.3 mm of Hg); IR (film) 2958, 1596, 1465, 1330, 1279,1248, 1083 cm⁻¹; ¹H-NMR (300 MHz, acetone-d₆) 7.15 (d, 1H, J=21.9 Hz),6.31 (d, 1H, J=21.9 Hz), 5.66 (dd, 2H, J=15.9, 1.2 Hz), 5.33 (dd, 2H,J=15.9, 6.6 Hz), 2.30-2.15 (m, 2H), 1.49 (s, 6H), 1.08 (s, 12H), 0.94(d, 6H, J=6.9 Hz), 0.93 (d, 6H, J=6.9 Hz), 0.10 (s, 9H); ¹³C-NMR (75.4MHz, acetone-d₆) 157.9, 135.7, 134.8, 93.0, 46.5, 32.3, 26.1, 22.90,22.86, 19.3, −1.6;

MS EI (m/e, relative intensity) (M+ 418)

EXAMPLE 14trans-2-(2-Isobutoxyethyl)-4,5-dimethyl-4,5-di[(E)-1,1,4-trimethyl-2-pentenyl]-1,3,2-dioxaborolane

This compound was prepared by the general method C from isobutyl vinylether (2.61 ml, 20 mmol) with the reaction mixture being stirred for afurther 2 hours at RT before addition of the 2,3-butanedione and wasisolated in a yield of 73% (6.11 g, 14.53 mmol) in the form of acolorless viscous oil; b.p. 120-130° C. (0.2 mm of Hg); IR (film) 2958,2870, 1466, 1382, 1322, 1107, 1082 cm⁻¹; ¹H-NMR (200 MHz, acetone-d₆)5.68 (dd, 2H, J=15.8, 1.1 Hz), 5.33 (dd, 2H, J=15.8, 6.8 Hz), 3.56 (t,2H, J=7.5 Hz), 3.15 (dd, 2H, J=6.6, 1.5 Hz), 2.35-2.15 (m, 2H),1.90-1.75 (m, 1H), 1.46 (s, 6H), 1.19 (t, 2H, J=7.5 Hz), 1.08 (s, 6H),1.07 (s, 6H), 0.96 (d, 12H, J=6.7 Hz), 0.87 (d, 6H, J=6.6 Hz); ¹³C-NMR(125.8 MHz, acetone-d₆) 135.3, 134.0, 92.2, 77.6, 67.7, 45.9, 31.8,28.8, 25.6, 25.5, 22.4, 19.34, 19.32, 18.8;

MS EI (m/e, relative intensity) (M+ 420)

EXAMPLE 15(E)-2-(6,6-Dimethylbicyclo[3.1.1]hept-2-ylmethyl)-4,5-dimethyl-4,5-bis(1,1,4-trimethylpent-2-enyl)-[1,3,2]dioxaborolane

This compound was prepared by the general method C from(1S)-(−)-β-pinene (3.15 ml, 20 mmol) with the reaction mixture beingstirred for another 1 hour at RT before addition of the 2,3-butanedioneand was isolated in a yield of 72% (6.56 g, 14.37 mmol) in the form of acolorless viscous oil; b.p. 135-150° C. (0.25 mm of Hg); IR (film) 2957,2904, 2868, 1466, 1374, 1315, 1083 cm⁻¹; ¹H-NMR (300 MHz, acetone-d₆)5.68 (dt, 2H, J=15.9, 1.2 Hz), 5.33 (ddd, 2H, J=15.9, 6.6, 1.5 Hz),2.40-2.07 (m, 6H), 2.03-1.80 (m, 4H), 1.60-1.45 (m, 1H), 1.45 (s, 6H),1.92 (s, 3H), 1.08 (s, 15H), 1.05-1.00 (m, 1H), 0.96 (d, 12H, J=6.6 Hz),0.94-0.85 (m, 1H); ¹³C-NMR (75.4 MHz, acetone-d₆) 135.90, 135.87, 134.4,92.5, 49.5, 49.4, 46.4, 46.3, 42.2, 39.5, 38.0, 34.61, 34.58, 32.39,32.35, 28.8, 27.46, 27.43, 26.44, 26.38. 26.1, 26.0, 25.8, 23.8, 22.94,22.93, 19.51, 19.50;

MS EI (m/e, relative intensity) (m+ 456)

1. A process for preparing boronic acid esters by reaction of a dienewith sodium borohydride in the presence of a first oxidant selected fromthe group consisting of an alkyl halide, a dialkyl sulfate, and mixturesthereof to form the corresponding bis(allyl)borane of the formula (I):

wherein R¹-R⁶ are H, aryl or substituted or unsubstituted C₁-C₄-alkyl ortwo of the radicals R¹-R⁶ may be closed to form a cyclic system, andfurther reaction of the borane (I) with an alkene (II) or alkyne (IV)to:

give the alkylbis(allyl)borane (III) or alkenylbis(allyl)borane (V):

wherein the radicals R⁷ to R¹² are: aryl, substituted or unsubstituted,alkyl-(C₁-C₈), which may be branched and/or substituted, alkoxy-(C₁-C₈),acyloxy-C₁-C₈), O-phenyl, fluorine, chlorine, NO₂, NH₂, NHalkyl-(C₁-C₈),Nalkyl₂-C₁-C₈), CN, CHO, SO₃H, SO₃R, SO₂NH₂, SO₂N(alkyl-(C₁-C₈))₂,SO₂-alkyl-(C₁-C₈), COO-alkyl-(C₁-C₈), CONH₂, CO-alkyl-(C₁-C₈), NHCHO,CF₃, 5-membered heteroaryl or 6-membered heteroaryl, where two orradicals R⁷ to R¹² may also form a cyclic ring system which may containheteroatoms and directly oxidizing the alkylbis(allyl)borane (III) oralkenylbis(allyl)borane (V) In the presence of a second oxidant to formthe corresponding bisallyl alkylboronate or alkenylboronate.
 2. Theprocess as claimed in claim 1, wherein the second oxidant is selectedfrom the group consisting of formaldehyde, acetone, flyoxal, diacetyl,and mixtures thereof.
 3. The process of claim 1, further comprisinghydrolyzing the boronic acid esters to form boronic acids.